Line protector for a communication circuit

ABSTRACT

A plug-in line protector for a communications circuit provides protection for over voltage and over current conditions of the line. A cup and slug module containing carbon blocks provides an air gap for grounding high voltage transients. For an over current condition, a solder pellet melts causing the cup and slug to engage due to spring pressure and ground the line through a grounding strip. The grounding strip has plastic insulation bonded to a portion thereof and interposed between the line and ground. In an over current condition heat from one of the carbon blocks is transferred through the cup to the metalic strip. This heat may cause the plastic to melt to provide a back-up or secondary low resistance path to ground. The protector may also have a resettable over current responsive mechanism in the form of a heat coil in the line. When the heat coil breaks, a printed circuit board, on which the heat coil is mounted, is moved by a volute spring. In over voltage and in over current conditions, the volute spring conducts current from the printed circuit board to the slug and cup and to ground.

United States Patent [191 Baumbach 1 Feb. 26, 1974 [75] inventor: Bertram W. Baumbach, Arlington Heights, 111.

[73] Assignee: Reliable Electric Company, Franklin Park, Ill.

[22] Filed: Oct. 6, 1971 [21] Appl. No.: 187,018

{52] US. Cl .L 337/32, 337/15, 3.37/19 [51] Int. Cl. H0lh 79/00 [58] Field of Search 337/28, 31, 32, 33, 34, 15, 337/19 [56] References Cited UNITED STATES PATENTS 1,726,807 9/1929 Cook 337/32 2,179,935 11/1939 Kayatt 337/19 3,587,021 6/1971 Baumbach 337/32 3,255,330 6/1966 MacKenzie 337/32 845,010 2/1907 Rolfe 337/32 2,219,705 10/1940 Jones; 337/32 Primary Examiner-Bernard A. Gilhean Assistant Examiner-F. E. Bell Bushnell T1571 ABSTRACT A plug-in line protector for a communications circuit provides protection for over voltage and over current conditions of the line. A cup and slug module containing carbon blocks provides an air gap for grounding high voltage transients. For an over current condition,

' a solder pellet melts causing the cup and slug to engage due to spring pressure and ground the line through a grounding strip. The grounding strip has plastic insulation bonded to a portion thereof and interposed between the line and ground. In an over cur rent condition heat from one of the carbon blocks is transferred through the cup to the metalic strip. This heat may cause the plastic to melt to provide a backup or secondary low resistance path to ground The protector may also have a resettable over current responsive mechanism in the form of a heat coil in the line. When the heat coil breaks, a printed circuit board, on which the heat coil is mounted, is moved by a volute spring. In over voltage and in over current conditions, the volute spring conducts current from the printed circuit board to the slug and cup and to ground. I

, 14 Claims, 12 Drawing Figures PATENIED FEB26 I974 SHEEI 2 [IF 3 LINE PROTECTOR FOR A COMMUNICATION CIRCUIT BACKGROUND OF THE INVENTION This inveniton relates to an improved line protector of the type shown in U. 8. Pat. No. 3,587,021, that issued June 22, 1971. Line protectors of the type with which the present invention is concerned are generally located between banks of telecommunications equipment. For instance, such protectors may be interposed between central office switching equipment and inside switching-related equipment. These protectors serve SUMMARY AND OBJECTS OF THE INVENTION The protector is typically designed for protection of two lines of a communications circuit. Each circuit is provided with protection against both over voltage and over current. When an over voltage of short duration is applied to either or both of, the incoming lines connected to the protector, the protector provides an arcgap path to ground so that the overvoltage does not reach inside equipment or central office equipment. When an over voltage fault is of prolonged duration, or an over current voltage is such that there is arcing across the arc-gap, the current passing through the arcgap generates sufficient heat to melt a fusible solder pellet. In this instance, the line is connected to ground through a T-bar ground strip which is, in turn, connected to the ground pin of the protector. The module in the protector containing the solder pellet can be removed and replaced to recondition the protector for subsequent normal operation after the over current fault has been corrected.

The module proper also contains contact members in the form of a cup and slug, both of metal of high conductivity, and both surrounded by an insulating sleeve to prevent possible arc-over between the adjacent mo ule for the other line. The cup rests on the metalic T-bar ground strip. ln addition, the T-bar strip has spaced apart strips or layers (one for each module) of insulating plastic interposed between the base of each cup and the remainder of the T-bar. The module also houses carbon blocks that provide the arc-gap. The solder pellet maintains the cup'and slug apart in opposition to the spring pressure tending to urge the cup and slug into engagement when the solder pellet melts. In one form of the invention the spring may be an extended part of the terminal'member that electrically connects the two pins of one line.

The T-bar strip also provides a secondary low.resistance path for grounding the line when such over current condition exists as to cause a sufficient heating of the carbon block that is in the cup. Under such conditions, heatfrom the carbon block melts the insulating plastic on the T-bar ground strip to provide a direct low resistance path to the ground pin. The T-bar is of a sufticiently yieldable yet highly conductive material that permits the T-bar to distend upon melting of the insulating plastic strip so as to bring the T-bar into engagement with the base of the contact member that is connected to a pin terminal of the line.

As in the aforesaid US. Pat. No. 3,587,021, one form of the present invention further provides against an over current fault in the line by the use of a heat coil assembly. Theheat generated in the coil as a result of excessive current melts a fusible material following which internal switching within the protector takes place to ground the line and also to actuate an alarm. Likewise, the arrangement provides for a visual indicating means for showing at a glance which line has been affected by the over current condition. Each line of a circuit pair in the protector has a protective circuit board for establishing the internal connections and for performing the aforesaid switching function, and each printed circuit member carries one of the heat coil assemblies that responds to the over current condition. Each heat coil assembly includes a latch or pawl and a cooperating ratchet wheel to effect the switching action. Upon melting of the heat coil, each printed circuit board is moved by a volute spring that is part of the aforesaid module containing the carbon blocks. The volute spring is particularly effective in this invention in that it has a relatively high current-carrying capacity because the volute spring has a greater cross section of material in the same space as a coil spring with comparable operating force and travel.

Accordingly, it is an objectof this invention to provide a line protector of the type stated which embodies a number of fail-safe modes of operation, the features of which are all in a single miniature enclosure, whereby metalic groundings will take place under all line-fault conditions.

A further object of this invention is to provide a line protector of the type stated which permits ready replacement of various major parts as may be required for in-service maintainance.

Another object of this invention is to provide a line protector that provides two identical arrestor assemblies in one housing and yet prevents interaction between the two assemblies under fault conditions.

ERIEF DESCRIPTION OF THE FIGURES FIG. 1 is a perspective view of a plugin type of line protector constructed in accordance with and embodying the present invention;

FIG. 2 is a front elevational view, partially broken away in section, and showing the protector in its normal operating condition;

FIG. 3 isan enlarged fragmentary portion of FIG. 2 but showing the condition of the protector following an over current fault in both lines;

FIG. 4 is a fragmentary sectional view taken approximately along line 4-4 of FIG. 2;

FIG. 5 is a front elevational view of the base and contact assembly of the protector of FIGS. l4 and which forms part of the present invention;

FIG. 6 is a fragmentary sectional view taken along line 6-6 of FIG. 5;

FIG. 7 is a perspective view of the T-bar ground contact strip which forms part of the present invention;

FIG. 12 is a sectional view taken approximately along line 1212 of FIG. 11 and showing the condition of the protector when one of the lines is subject to an over current condition that actuates the heat coil therein.

In each of the sectional views, certain of the parts are shown in elevation and some parts not normally seen in the section plane are shown, all for clarity in illustrating the invention.

DETAILED DESCRIPTION gated detachable housing 16 of generally rectangular cross section. The base 15 and housing 16 are fabricated of a suitable dielectric material, such as molded plastic. The housing 16 maybe secured to the base 15 by bosses 17 which snap into correspondingly shaped openings in the side walls of the housing 16. The upper end of the housing 16, namely the remote from the base 15, has a neck portion 18 terminating in a flange 19,by which the protector may be gripped for removal from the placement into a receptacle. The neck portion 18 defines an opening 20 for use in connection with components of the protector used in the form of the invention shown in FIGS. 8-12. The particular housing construction shown in connection with the form of the inveniton of FIGS. 1-4, especially as concerned the length of the housing and the opening 20, is illustrated simply to point out that the housings of the two forms of the invention may be of generally similar construction.

Mounted in and projecting outwardly from the base 15 is a series of'conductive plug-in terminal pins identified by the reference numerals 25-31 inclusive. In the form of the invention shown, there is a first line pin 25, a first central office pin 29, a second line pin 26, a second central office pin 30, a polarizing pin 28, and an alarm pin 31. The alarm pin 31 is shown in FIG. 1 but is not used in the form of the invention shown in FIGS. 14. However, alarm pin 31 is used in the form of the invention shown in FIGS. 8-12. The irst line pin 25 and the first central office pin 29 are components in one of the lines through the protector whilethe second line pin 26 and second central office pin 30 are components in the other line through the protector 10. The ground pin 27 (and alarm pin 31 when used as described hereafter) are common to both lines while the polarizing pin 28 is an unconnected or dummy pin that assures proper orientation of the protector unit when it is plugged into its receptacle.

Referring now to FIGS. 2-4, it will be seen that pins 25 and 29 are electrically and mechanically connected by contact member 35, which is formed of flat springlike material. As best shown in FIG. 4, the connection between the pins 25, 29 is through a generally channel shaped base portion 37 of the contact member 35, the channel shaped portion 37 receiving an elongated rib 38 on the base and having opposed flanges 39, 41 at which the pins 29, 25 are staked. In like manner,

contact member 42, which is identical to contact member 35, connects the pin 26 with the pin 30.

leg 48 is centrally of the strip 45 and depends from one edge thereof. On opposite sides of the leg 48 the strip 45 has spaced insulating strips 52, 52 laminated thereto. The insulating strips 52, 52 may be tapes of a polyethylene terephthalate resin, commonly sold under the trademark Mylar. As willbest be seen in FIGS.:

2, 4 and 6, the insulating strips 52, 52 are disposed against the channel shaped portions 37 of the respective contact members 35, and for purposed presently more fully appearing.

For each of the two lines there is provided a removable and replacable module 60 for providing a ground to the ground pin 27 in the event of an over voltage or an over current condition. The modules are identical and each includes contact members in the form of a slug 62 and cup 64, both being of copper or other highly conductive material. The slug 62 and cup 64 are surroundedby an insulating sleeve 66. Within the cup 64 is a carbon arc-gap assembly comprising carbon blocks 68, and an insulating member 72. The carbon block 68 is'suitably bonded to the insulating member 72, the latter bearing at one end against the carbon block 70 to constitute a spacer so that an air gap 74 is established between the carbon blocks 68, 70. The air gap 74 is determined by the minimum arc-over voltage for thich the unit is designed. A fusible solder pellet 76 is interposed between the carbon block 68 and the as-.

sociated slug 62 so that in the normal operating condition of the unit, the slug 62 and associated cup 64 are maintained spaced apart.

The upwardly extended parts of the respective contact members 35, 42 lie in grooves 78 (FIG. 4) fomied in a sidewall of the housing 16. Each of these upwardly extending parts of the contact members 35,

42 terminates in a downwardly extending generally V shaped portion 80 which bears firmly against the outer end of the associated slug 62. Thus, each contact member 35, 42 constitutes a spring tending to urge the slug 62 toward the cup 64; however, engagement of the cup 64 and slug 62 is prevented, in the normal operation of the protector, by the solder pellet 76.

When an over voltage condition of relatively short duration is applied to one of the lines, for example at either pin 25 or at pin 29, the contact 35 provides a conductive path through its upper V" shaped end 80, through slug 62, solder pellet 76 and to carbon block 68. This over voltage transient will arc across gap 74 to carbon block 70 and will be grounded through the strip 45 to the ground pin 27. Under such conditions the solder pellet 76 does not melt. The components of the protector remain in the conditions shown. and FIGS. 2 and 4, and nothing need be done to restore the protector to its normal operating condition. If an over voltage condition appears on the other line, namely the line in which pins 26 and 30 are located, the ground is through the contact 42 and the module 60 associated therewith, all as previously described.

If an over current condition appears in one of the lines due to a prolongedvoltage that is above arcing voltage, the protector will provide a fail-safe operation.

Assuming that such over current conditions appears on the line containing pin 25 and pin 29 (which are connected through contact 35) the current through the associated module 60 will cause the solder pellet 76 to melt, as shown with respect to the left hand module 60 in FIG. 3. When the solder pellet 76 melts the spring force applied by the contact35 forces the slug v62 into engagement with the cup 64 whereby a ground connection to the ground pin 27 is made through the slug 62, cup 64 and conductive T-bar strip 45. The base of the cup 62 is pressed firmly against the grounding strip 45 by reason of the spring pressure from the contact 35 so as to insure a low resistance flow path for the current to ground. If an over current condition as aforesaid, appears in the line containing the pins 26, 30, aground connection through to the pin 27 will result in the manner just described except, of course, that the ground will be through the contact member 42 and its associated module 60.

The present invention also contemplates a secondary fail-safe grounding path through the conductive strip 45. Such condition is shown with respect to the contact 42 and the right hand module 60 in FIG. 3, it being understood that the left hand module will function in like manner. In some instances'the over current condition causes a heating of the carbon block 70 which'transmits heat through the base of the cup 62 to the conductive strip 45 and the plastic insulating strip 52 associated therewith. If the heat from the carbon block 70 is sufficient in terms of magnitude and duration, the plastic insulating strip 52 will melt causing a direct, low resistance, very short path for shunting current to the ground pin 27. In this regard it should be noted that in an over current'condition the solder pellet 76 will generally melt to provide the ground connection. However, conditions may arise where despite an arc-over voltage, the currents are relatively low and are of a long duration, in which case it is possible that the solder pellet 76 might not melt soon enough to prevent the protector from destroying itself. In such case, the heating of the carbon block 70 will melt the plastic insulating strip 52 and cause the conductive strip 45 to be pressed into engagement with the channel shaped portion 37 of the contact member 35 or 42, as the case may be, to form the ground connection. For this purpose strip is highly flexible so that it will yield under pressure applied from a contact member 42, as shown with respect to the right hand module in FIG. 3. In'some over current conditions, it is possible that the solder pellet will melt and the insulating strip 52 will also melt to providetwo shunt circuits to ground.

Each of the module 60 is fabricated as a selfcontained unit. Consequently, when an over current condition causes either of the module 60 to respond and melt the solder pellet 76, the module 60 may be readily replaced by simply removing the housing 16, withdrawing the spent module, and inserting a new module in its place. Since the insulating strip 52 does not melt in every over current condition, replacement of the strip 45 is frequently not necessary However, if that is required, a new base unit 15 with assembled pins and contacts, such is shown in FIGS. 5 and 6 can be readily assembled with one or more new modules 16 and then further assembled with the housing 16.

FIGS. 8-12 show a modified form of the invention in I which like reference numerals indicate like parts previously described with respect to the form of the invention shown in FIGS. 1-7. Referring first to FIG. 9, it will be seen that module 60a includes slug 62a, cup 64a,

carbon blocks 68a, 70a, insulating member 72a and fusible solder pellet 76a. The arrangement is generally similar to that of the module 60, previously described, except for certain dimensions of the parts. For instance, the carbon block 70a is shorter than the block 70 and the slug 62a is somewhat shorter than the slug 62. Seated on the upper surface of the slug 62a is a volute spring 84, for purposes presently more fully ap pearing.

Referring now to FIGS. 11 and 12, it will be seen that line pin 25 is mechanically and electrically connected to the base of terminal 86 which extends upwardly along the inside of the housing 16 for engagement with a printed circuit 88 on a printed circuit board 90 of insulating material. The printed circuit 88 extends downwardly to the bottom edge of the printed circuit board 90 and across the latter and upwardly for a short length 91 on the opposite side of the printed circuit board 90. The part of the printed circuit 88 along the lower edge of the 'board 90 is in electrical and mechanical contact with the upper end of the conductive volute spring 84. The upper end of the printed circuit board 90 :has an extension 93that projects through the opening 20 in the neck 18 of the housing. The line pin 29 is mechanically and electrically connected to an additional contact 87 which is similar in construction to the contact 86 and extends upwardly withinthe housing 16 for contact with a printed circuit 95 on the printed circuit board 90. Electrical connection is established between the two printed circuits 88, 95 through a heat coil assembly/-97, which may be of the general type shown in the aforesaid U. S. Pat.No. 3,587,021.

Line pins 26 and 30 are connected respectively to identical contacts 99 (one being shown in FIG. 12), the contacts 99 also being identical to the contacts 86, 87. The respective contacts 99 engage printed circuits 88a and 95a (FIG. 11) onprinted circuit board 90a, all being similar to the corresponding element 88, 90, 95, previously described. Printed circuit boards 90, 90a are spaced apart as shown in FIG. 12. Likewise, circuit board 90ahas a reduced extension 930 that is similar to the extension 93. Also, electrical connections to the two printed circuits 88a, 95a is through a heat coil assembly 97a, which is identical to the heat coil assembly 97. The volute spring 84 in the left hand module 60a, FIG. 12, makes electrical and mechanical contact with the lower edge of the printed circuit 88a.

Ground pin 27 and alarm pin 31 are alsoconnected to contacts 101 (one being shown in FIG. 12) for engagement with the circuit board 90, 90a. However, in

the normal operating condition of the protector, the

contacts 101 engage the circuit board 90, 90a at regions outside of the printed circuits thereon. So that the contacts 101 may engage both circuit boards, the upper ends thereof are bifurcated as best seen in FIG. 12. The remaining contacts 86, 87, 99 previously described, may also be bifurcated.

The heat coil assembly 97 includes a metal sleeve extending through the printed circuit board 90 and being pressfitted or otherwise suitably held for a nonrotatable fit therein. A length of resistance wire 182 has one end connected electrically to the printed circuit 88, the wire 182 also being wound about the sleeve 180. The other end portion of the wire 182 passes through an aperture 184 to the opposite side of the printed circuit board 90, where that other end of the wire is connected electrically to the printed circuit 95.

A similar arrangement is provided with respect to the companion heat coil assembly 97a.

A rotatable shaft 190 extends through the sleeve 180 and on the front side of the printed circuit board 90 the shaft 190 carries a ratchet wheel 192, the latter being spaced from the front surface of the printed circuit board 95. The other printed circuit board 90a has a similar rotatable shaft 190a and a ratchet wheel 192a.

A layer of fusible material (not shown) is located between the metal sleeve 180, 180a and the associated rotatable shaft 190, 190a. When the fusible material is in its unheated or solidcondition, shafts 190 and 190a are held rigidly within the associated. sleeve 180, 180a. However, when an over current is present in a circuit in the protector, the heat coil of that circuit develops sufficient heat to melt the fusible material permitting the associated ratchet wheel to rotate. When this occurs, the printed circuit board 90 or 90a, as the case may be, will be moved outwardly by the force of the associated volute spring 84. FIG. 12 shows the condition of the printed circuit board 90a when the fusible material in the heat coil assembly 97a has melted due to an over current condition. Then the volute spring 84 expands so that the extention'93a projects a substantial distance above the end of the protector, giving a visual indication that an over current condition has occurred in the circuit or line containing the pins 26 and 30. It should also be noted that under such'conditions the contact 101 to the ground pin 27 will engage the printed circuit 88a to ground the line. Likewise, the contact to the alarm pin 31 will engage the printed circuit 88a through the portion thereof that is on the opposite side of the circuit for 90a. Such portion is not specifically shown butis identical with the printed circuit portion 91 shown in FIG. 11. The alarm pin 31 may be in a circuit which contains a suitable audible or visual alarm in the central office.

In the normal condition of the operation of the protector, the printed circuit boards 90, 90a are held in their respective inward positions against the forces of the volute springs 84, .84 by a latch arrangement that includes a latch support 194 having slots 195, 196 therein. The sleeves 180, 180a extend through the slots 195, 196 so that the slots permit relative movement between the latch support 194, which remains stationary and one or both of the printed circuit boards 90, 90a.

the upper enclosure wall of the housing-l6.

' The central portion of the latch support 194 carries a U-shaped latch I98 which has two tines 200(FIG.

'l I) for respective engagement with the ratchet wheels 192, 192a. Thus, the latch'l98 maintains the printed circuit boards in their normal operating conditions in the protector in opposition to the forces of the volute springs 84, 84. However, when one or both of the ratchet wheels are permitted to rotate due to softening of the fusible material in one or more of the heat coil assemblies 97, 97a one or both of the volute springs 84, 84, as the case may be, permits one or more of the ratchet wheels to rotate, thereby moving the associated printed circuit board upwardly as shown with respect to the circuit board 90a in FIG. 12.

When a heat coil circuit is broken as aresult of an over current condition, the fusible material in the heat coil assembly solidifies rapidly since the current flow in the'circuit has stopped. Accordingly, a rigid connection will then be established between, for example, the shaft basegmeans forming a conductive connection between The upper ends of the latch" support 194 bear against 7 ratchet wheel 192a to a non-rotatable condition. The protector may then be reset by pushing in on the extension 93a, 'depressing'the volute spring 84, and moving the ratchet wheel 192a, until it comes into final latching engagement with the associated line of the latch 198.

As in the form of the invention shown in FIG. 1-7 the protectors of FIGS. 8-12, respond to an over voltage transient by an arc across air-gap 74 for grounding through the base of cup 64a and conductive strip 45, which is connected to ground pin 27. Likewise, an over voltage of long duration will melt the solder pellet 76a whereby the volute spring 84 will force the slug 62a into contact with the upper edge of the cup 64a to esbase of the cup 64a, melting the insulating strip 52 to provide a secondary grounding path for fail-safe operation. i

, The inventionis elaimedas follows;

' 1. In a line protector for communications circuitry,

said protector having a base of insulating material, first and second conductive terminals mounted on said said first and second terminals to provide a line circuit through said protector, a conductive ground terminal, and a shunt circuit for over voltage and over current fault conditions in said line circuit; said shunt circuit including normally spaced apart contact members, one of said contact members being in conductive connection with said ground terminal, spaced blocks defining an air gap, springmeans urging said contact members toward their engaged positions, and a solder element that normally maintains said contact members apart in opposition to said spring means, whereby an over voltage of short duration at either of said first'or second terminals produces an are at said air gap. and current flow therethrough to said ground terminal, and an over voltage of longer duration causes the solder element to melt and causes said spring means to urge said contact members into engagement to provide a current path to said ground terminal; a grounding strip conductively connected to said ground terminal and to .one of the contact members and forming part of said shunt circuit, and an insulating strip interposed between said line circuit and said grounding strip for melting upon heating provide an auxiliary, low' resistance, short current path from said line circuit through said grounding strip and to said ground terminal.

2. Ina line protector according to claim 1, said insulating strip being bonded to said grounding strip.

3. In a line protector according to claim 2, said one contact member being seated on said grounding strip;

4. In a line protector according to claim 1, said one contact member, being a cup-shaped piece that contains one of said spaced blocks such that heating of said one spaced block in turn heats said contact member in said over current condition.

5. In a line protector according to claim 1, said one contact member being amp and the other contact member being a slug, an insulating sleeve surrounding said cup and slug, and said spaced blocks and solder el- 9 ement being within said insulating sleeve, the slug being movable in said sleeve upon melting of said solder ele ment.

6. in a line protector according to claim 1, said grounding strip being a generally T-shaped piece with the insulating strip being bonded thereto.

7. In a line protector according to claim 1, said spring means being a contact constituting said means forming the conductive connection between said first and second terminals.

8. In a line protector according to claim 1 said spring being a volute spring.

9. In a line protector according to claim 8, said means forming a conductive connection between said first and second terminals including aboard having printed circuits thereon and a heat responsive mechanism operable to break said conductive connection of said line circuit in an over current condition thereof, said volute spring engaging said printed circuit and imposing force on said board. 7

10. In a line protector for communication circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a

blocks defining an air gap, contact members in conductive connection with said blocks, spring means urging said contact members together, and a solder element maintaining said contact members apart in opposition to said springmeans, one of said contact members housing one of said blocks and engaging said grounding strip so'that heat from said one block may be transmitted through said one contact to said' insulating strip.

11. in aline protector according to claim 10 said spring means including -a volute spring, said protector also including a circuit board of insulating material with printed circuits thereon, said printed circuits being in said line circuit, and said volute spring being in said other shunt circuit and engaging said board and being in conductive connection with a printed circuit thereon.

12. In a line protector according to claim 10, said spring means comprising a metallic portion forming said conductive means, said spring means having opposed end portions with said contact members con fined therebetween. a

13. In a line protector for communications circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a ground terminal, and a shunt circuit from said line circuit to said ground terminal and responsive to an over current fault at either of said circuit terminals, said shunt circuit including a grounding strip and an insulating strip laminated thereto, said insulating strip normally interrupting said shunt circuit, and means adjacent to said insulating strip and responsive to an over current condition in said line circuit for melting said insulation strip and closing said shunt circuit; said means adjacent to said strip also includes a spring-pressed member in another shunt circuit from said line circuit to said ground terminal for urging said grounding strip into electrical contact with said conductive means that connects said line terminals to provide a lowresistance short current path from the line circuit to said groun terminal.

14. In a line protector for a communications circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a ground terminal, and a shunt circuit from said line circuit to said ground terminaland responsive to an over current fault at either of said circuit terminals, said shunt circuit including a grounding strip and an insulating strip laminated thereto, said insulating strip normally interrupting said shunt circuit, and means ad jacent to said insulating strip and responsive to an over current condition in said line circuit for melting said in sulation strip and closing said shunt circuit; said means adjacent to said strip also including a component in another shunt circuitfrom said line circuit to said ground terminal and responsive to an over current fault in said line for heating said insulating strip. 

1. In a line protector for communications circuitry, said protector having a base of insulating material, first and second conductive terminals mounted on said base, means forming a conductive connection between said first and second terminals to provide a line circuit through said protector, a conductive ground terminal, and a shunt circuit for over voltage and over cuRrent fault conditions in said line circuit; said shunt circuit including normally spaced apart contact members, one of said contact members being in conductive connection with said ground terminal, spaced blocks defining an air gap, spring means urging said contact members toward their engaged positions, and a solder element that normally maintains said contact members apart in opposition to said spring means, whereby an over voltage of short duration at either of said first or second terminals produces an arc at said air gap and current flow therethrough to said ground terminal, and an over voltage of longer duration causes the solder element to melt and causes said spring means to urge said contact members into engagement to provide a current path to said ground terminal; a grounding strip conductively connected to said ground terminal and to one of the contact members and forming part of said shunt circuit, and an insulating strip interposed between said line circuit and said grounding strip for melting upon heating due to an over current condition in the line circuit to provide an auxiliary, low resistance, short current path from said line circuit through said grounding strip and to said ground terminal.
 2. In a line protector according to claim 1, said insulating strip being bonded to said grounding strip.
 3. In a line protector according to claim 2, said one contact member being seated on said grounding strip.
 4. In a line protector according to claim 1, said one contact member being a cup-shaped piece that contains one of said spaced blocks such that heating of said one spaced block in turn heats said contact member in said over current condition.
 5. In a line protector according to claim 1, said one contact member being a cup and the other contact member being a slug, an insulating sleeve surrounding said cup and slug, and said spaced blocks and solder element being within said insulating sleeve, the slug being movable in said sleeve upon melting of said solder element.
 6. In a line protector according to claim 1, said grounding strip being a generally T-shaped piece with the insulating strip being bonded thereto.
 7. In a line protector according to claim 1, said spring means being a contact constituting said means forming the conductive connection between said first and second terminals.
 8. In a line protector according to claim 1 said spring being a volute spring.
 9. In a line protector according to claim 8, said means forming a conductive connection between said first and second terminals including aboard having printed circuits thereon and a heat responsive mechanism operable to break said conductive connection of said line circuit in an over current condition thereof, said volute spring engaging said printed circuit and imposing force on said board.
 10. In a line protector for communication circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a ground terminal, and a shunt circuit from said line circuit to said ground terminal and responsive to an over current fault at either of said circuit terminals, said shunt circuit including a grounding strip and an insulating strip laminated thereto, said insulating strip normally interrupting said shunt circuit, and means adjacent to said insulating strip and responsive to an over current condition in said line circuit for melting said insulation strip and closing said shunt circuit; said means also including another shunt circuit that includes blocks defining an air gap, contact members in conductive connection with said blocks, spring means urging said contact members together, and a solder element maintaining said contact members apart in opposition to said spring means, one of said contact members housing one of said blocks and engaging said grounding strip so that heat from said one block may be transmitted through said one contact to said insulating strip.
 11. In a line protector according to claim 10 said spring means Including a volute spring, said protector also including a circuit board of insulating material with printed circuits thereon, said printed circuits being in said line circuit, and said volute spring being in said other shunt circuit and engaging said board and being in conductive connection with a printed circuit thereon.
 12. In a line protector according to claim 10, said spring means comprising a metallic portion forming said conductive means, said spring means having opposed end portions with said contact members confined therebetween.
 13. In a line protector for communications circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a ground terminal, and a shunt circuit from said line circuit to said ground terminal and responsive to an over current fault at either of said circuit terminals, said shunt circuit including a grounding strip and an insulating strip laminated thereto, said insulating strip normally interrupting said shunt circuit, and means adjacent to said insulating strip and responsive to an over current condition in said line circuit for melting said insulation strip and closing said shunt circuit; said means adjacent to said strip also includes a spring-pressed member in another shunt circuit from said line circuit to said ground terminal for urging said grounding strip into electrical contact with said conductive means that connects said line terminals to provide a low resistance short current path from the line circuit to said ground terminal.
 14. In a line protector for a communications circuitry, said protector having a line circuit with terminals and conductive means connecting said line terminals, a ground terminal, and a shunt circuit from said line circuit to said ground terminal and responsive to an over current fault at either of said circuit terminals, said shunt circuit including a grounding strip and an insulating strip laminated thereto, said insulating strip normally interrupting said shunt circuit, and means adjacent to said insulating strip and responsive to an over current condition in said line circuit for melting said insulation strip and closing said shunt circuit; said means adjacent to said strip also including a component in another shunt circuit from said line circuit to said ground terminal and responsive to an over current fault in said line for heating said insulating strip. 